Lipid vesicles, or liposomes, have demonstrated utility for delivering therapeutic agents and diagnostic agents to target tissues and organs. Lipid vesicles have an aqueous interior enclosed by one or more lipid bilayers, where the therapeutic agent is entrapped in the aqueous interior spaces or within the lipid bilayer. Thus, both water-soluble and water-insoluble drugs can be transported by lipid vesicles within the aqueous spaces and the lipid bilayer, respectively.
The action of many drugs involves their direct interaction with sites inside the cell. For action, the drug must pass through the cell membrane to reach the cytoplasm. Success in achieving intracellular delivery of a liposome-entrapped agent has been limited for a variety of reasons. One reason is that liposomes, after systemic administration to the bloodstream, are rapidly removed from circulation by the reticuloendothelial system. Another reason is the inherent difficulty in delivering a molecule, in particular a large and/or a charged molecule, into the cellular cytoplasm and/or the nucleus.
The limitation of rapid uptake by the reticuloendothelial system has largely been overcome by the addition of a hydrophilic polymer surface coating on the liposomes to mask the vesicle from recognition and uptake by the reticuloendothelial system. The extended blood circulation lifetime of liposome having a coating of polyethyleneglycol (PEG) polymer chains (U.S. Pat. No. 5,013,556) allows for a greater opportunity for uptake by a cell.
Delivery of charged molecules intracellularly remains a technical challenge. In particular, delivery of nucleic acids, both DNA and RNA, has been challenging, due to the charge and size of the molecules. Proteins, peptides, and charged drug compounds involve the same technical hurdle of transport across a cell membrane. One approach to delivery of negatively charged agents, particularly nucleic acids fragments for gene therapy, has been to complex the DNA or RNA with a cationic lipid. Electrostatic interaction of the cationic lipid with the nucleic acid permits formation of lipid-nucleic acid particles in a size range suitable for in vivo administration. The positively charged cationic lipid on the outer particle surfaces is beneficial for interaction with negatively-charged cellular membranes, to promote fusion or uptake of the lipid-nucleic acid particles into the cell.
However, the presence of the positive charge on the external surface of lipid particles prepared with cationic lipids is detrimental to the goal of achieving a long blood circulation lifetime for widespread biodistribution. The charge on the particles causes immediate binding with the tissue surfaces at or near the site of administration, substantially limiting the availability of particles for circulation and distribution to the target site. It would be desirable to design a lipid vesicle composition that is neutral upon administration to permit biodistribution, yet that is charged after a period of time, i.e., after biodistribution of the particles, to permit interaction with cell membranes for binding and intracellular delivery of the entrapped agent.